Date:   Tue, 8 Mar 2022 14:04:53 -0500
From:   Alex Deucher <alexander.deucher@....com>
To:     <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <corbet@....net>, <hpa@...or.com>, <x86@...nel.org>,
        <dave.hansen@...ux.intel.com>, <bp@...en8.de>, <mingo@...hat.com>,
        <tglx@...utronix.de>, <joro@...tes.org>,
        <suravee.suthikulpanit@....com>, <will@...nel.org>,
        <iommu@...ts.linux-foundation.org>
CC:     Alex Deucher <alexander.deucher@....com>
Subject: [PATCH] Documentation: x86: add documenation for AMD IOMMU

Add preliminary documenation for AMD IOMMU.

Signed-off-by: Alex Deucher <alexander.deucher@....com>
---
 Documentation/x86/amd-iommu.rst   | 85 +++++++++++++++++++++++++++++++
 Documentation/x86/index.rst       |  1 +
 Documentation/x86/intel-iommu.rst |  2 +-
 3 files changed, 87 insertions(+), 1 deletion(-)
 create mode 100644 Documentation/x86/amd-iommu.rst

diff --git a/Documentation/x86/amd-iommu.rst b/Documentation/x86/amd-iommu.rst
new file mode 100644
index 000000000000..89820140fefa
--- /dev/null
+++ b/Documentation/x86/amd-iommu.rst
@@ -0,0 +1,85 @@
+=================
+AMD IOMMU Support
+=================
+
+The architecture spec can be obtained from the below location.
+
+https://www.amd.com/system/files/TechDocs/48882_IOMMU.pdf
+
+This guide gives a quick cheat sheet for some basic understanding.
+
+Some Keywords
+
+- IVRS - I/O Virtualization Reporting Structure
+- IVDB - I/O Virtualization Definition Block
+- IVHD - I/O Virtualization Hardware Definition
+- IOVA - I/O Virtual Address.
+
+Basic stuff
+-----------
+
+ACPI enumerates and lists the different DMA engines in the platform, and
+device scope relationships between PCI devices and which DMA engine controls
+them.
+
+What is IVRS?
+-------------
+
+The architecture defines an ACPI-compatible data structure called an I/O
+Virtualization Reporting Structure (IVRS) that is used to convey information
+related to I/O virtualization to system software.  The IVRS describes the
+configuration and capabilities of the IOMMUs contained in the platform as
+well as information about the devices that each IOMMU virtualizes.
+
+The IVRS provides information about the following:
+- IOMMUs present in the platform including their capabilities and proper configuration
+- System I/O topology relevant to each IOMMU
+- Peripheral devices that cannot be otherwise enumerated
+- Memory regions used by SMI/SMM, platform firmware, and platform hardware. These are
+generally exclusion ranges to be configured by system software.
+
+How is IOVA generated?
+----------------------
+
+Well behaved drivers call pci_map_*() calls before sending command to device
+that needs to perform DMA. Once DMA is completed and mapping is no longer
+required, device performs a pci_unmap_*() calls to unmap the region.
+
+The AMD IOMMU driver allocates a virtual address per domain. Each PCIE
+device has its own domain (hence protection). Devices under p2p bridges
+share the virtual address with all devices under the p2p bridge due to
+transaction id aliasing for p2p bridges.
+
+IOVA generation is pretty generic. We used the same technique as vmalloc()
+but these are not global address spaces, but separate for each domain.
+Different DMA engines may support different number of domains.
+
+
+Fault reporting
+---------------
+When errors are reported, the DMA engine signals via an interrupt. The fault
+reason and device that caused it with fault reason is printed on console.
+
+See below for sample.
+
+
+Boot Message Sample
+-------------------
+
+Something like this gets printed indicating presence of the IOMMU.
+
+	iommu: Default domain type: Translated
+	iommu: DMA domain TLB invalidation policy: lazy mode
+
+
+PCI-DMA: Using AMD IOMMU
+------------------------
+
+Fault reporting
+^^^^^^^^^^^^^^^
+
+::
+
+	AMD-Vi: Event logged [IO_PAGE_FAULT domain=0x0007 address=0xffffc02000 flags=0x0000]
+	AMD-Vi: Event logged [IO_PAGE_FAULT device=07:00.0 domain=0x0007 address=0xffffc02000 flags=0x0000]
+
diff --git a/Documentation/x86/index.rst b/Documentation/x86/index.rst
index f498f1d36cd3..15711134eb68 100644
--- a/Documentation/x86/index.rst
+++ b/Documentation/x86/index.rst
@@ -22,6 +22,7 @@ x86-specific Documentation
    mtrr
    pat
    intel-iommu
+   amd-iommu
    intel_txt
    amd-memory-encryption
    pti
diff --git a/Documentation/x86/intel-iommu.rst b/Documentation/x86/intel-iommu.rst
index 099f13d51d5f..4d3391c7bd3f 100644
--- a/Documentation/x86/intel-iommu.rst
+++ b/Documentation/x86/intel-iommu.rst
@@ -1,5 +1,5 @@
 ===================
-Linux IOMMU Support
+Intel IOMMU Support
 ===================
 
 The architecture spec can be obtained from the below location.
-- 
2.35.1

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